One embodiment of the invention relates to a method for manufacturing quartz glass as well as to an evaporator adapted to this method.
To manufacture high-purity synthetic quartz glass, use is made of halogen-containing and non-halogen silicon compounds as starting materials. Halogen-containing feed materials, such as silicon tetrachloride (SiCl4) are to disadvantage in that corrosive acids, e.g., hydrochloric acid (HCl), are produced as side products when they are used. For this reason, use is increasingly made of non-halogen materials wherein, presently, alkyl polysiloxanes are at the center of interest.
According to U.S. Pat. No. 5,043,002, polymethyl cyclosiloxanes such as hexamethyl cyclotrisiloxane (HMCTS), octamethyl cyclotetrasiloxane (OMCTS) and decamethyl cyclopentasiloxane (DMCTS), are particularly adapted to manufacture high-purity silicon dioxide for optical fibres. The siloxanes are oxidized in a burner flame in the presence of oxygen to obtain SiO2 which accumulates in the form of fine amorphous particles that are referred to as silica soot. The SiO2 soot is collected and melted to form a glass which can be used as starting material for fibre-optic blanks or for optical components.
U.S. Pat. No. 5,356,451 discloses evaporators which are supposed to be adapted to evaporate halogen-containing and non-halogen silicon compounds. The liquid to be evaporated is applied to an inclined surface in a thin layer. The surface is heated, with the result that the liquid evaporates while it flows down the surface.
Polymethyl cyclosiloxanes are to disadvantage in that they tend to form resins and gels which result in the contamination of heat exchanger surfaces and burners as well as in the clogging of pipelines. For example, resins and gels are formed while the hydrolytic opening of the cyclic alkyl polysiloxanes to form linear hydroxyl-terminated siloxanes (silanols) is in progress, which have a considerably lower volatility than the cyclic compounds and deposit in the system. The silanols are reactive and react with cyclosiloxane molecules to form gel-like polymerisation products. It is also possible that silanol traces are contained as contaminants in the starting material.
EP 0 719 575 A2 discloses an evaporator for non-halogen silicon compounds, wherein gels are collected in a sump in order to prevent contamination of evaporator surfaces and clogging of the pipelines. The evaporator comprises a vertically aligned evaporation chamber into which the substances to be evaporated are sprayed. Due to the pressure drop, a part of the pre-heated liquid evaporates when it enters the evaporator, while another part evaporates when it impinges on the heater evaporator walls. The gels formed during evaporation are collected in the lower part of the evaporator and removed periodically. The nozzles for spraying the liquid are arranged such that they do not carry along the gels having accumulated in the sump of the evaporator.
According to U.S. Pat. No. 6,312,656, the formation of gels is supposed to be prevented in that the alkyl polysiloxanes are not evaporated but are directly sprayed into the burner flame in liquid form. The temperature load associated with evaporation and favouring the formation of gels is prevented in this manner.
U.S. Pat. No. 5,879,649 discloses alkyl polysiloxanes having a boiling point of less than 250° C., the alkyl polysiloxanes containing less than 14 ppm of high boiling contaminants having boiling points in excess of 250° C. These contaminants can be evaporated only poorly due to their high boiling points and accumulate in the evaporator where they react to form gel-like deposits. The alkyl polysiloxanes are purified by distillation and subsequent filtration of the distillate through activated charcoal and molecular sieves.
US 2012/0276291 discloses a method for evaporating alkyl polysiloxanes according to which the liquid to be evaporated is directed to the vertical walls of an evaporation chamber. The walls of the evaporation chamber are heated to such a high temperature that a part of the liquid evaporates. The remaining liquid flows along the walls and to the bottom of the chamber where it is removed continuously. Gels formed in the evaporator are rinsed out of the chamber along with this liquid.
Despite numerous efforts, the formation of gel is still a severe problem in the manufacture of high-purity quartz glass. The gels can concentrate in the evaporator and in the pipelines and impair the process stability. This is particularly applicable for the manufacture of glass fibres because, here, minute irregularities considerably impair the further processing of the glass blank to form fibres. In addition, the purification of the starting materials and the cleaning of evaporators and units are subject to substantial effort.